The present invention relates to a motor vehicle parking brake control device.
With today""s vehicle transmissions (synchronized, power-shifting or automatic transmissions), the parking brake is typically set by means of a mechanical rod assembly that blocks rotational movement of the transmission gears or the vehicle wheels by means of positive fit. In connection with improving ease of use, gears have been configured with electrically actuated gear switches, so that the change gear command can be performed by the driver by means of a push-button. Such a servo control could also be used in connection with the parking brake. With the use of continuously variable speed transmissions, which can realize a non-positive stationary state by adjusting to a infinitely high transmission ratio, the transition from the non-positive stationary state into the park position (and vice versa) is done without interrupting the braking effect of the vehicle. Thus, a goal is have a parking brake (mechanical, positive-fit locking of a drive-side gear) which can be locked and unlocked by transmission of an electrical command.
Because the operational safety of the vehicle depends on the function of the parking brake to a very large degree, the construction and design should be operationally safe, that is to say, redundant. Above all, this concerns the reliable execution of driver wishes for locking or unlocking, and preventing accidental locking while driving and accidental unlocking in a stationary state (e.g., through loss of auxiliary power, short circuit in the power circuit, and the like). Therefore, the electrical control unit should provide a return notification about the current state of the parking brake at all times.
To recognize the parking brake state, at least the two end positions of the parking brake, xe2x80x9clockedxe2x80x9d and xe2x80x9cunlocked,xe2x80x9d must be detected. Path sensors or end switches could be used, wherein they detect the position of the parking brake activation mechanism, and when each end position is reached, a contact is closed. However, because the activating mechanism end positions are strongly dependent on tolerances and wear, the corresponding signals cannot be triggered with sufficient accuracy. Reliability of path sensors can be adversely affected by abrupt movements of the activating mechanism. If an electrical parking brake is to be retrofitted, it is difficult to install path sensors or end switches into existing transmission housings without changing the existing transmission structure.
In general, the parking brake contains a detent with teeth that can engage the teeth of a gear of the drive train, so that a positive fit is produced and rotational movement of the drive train gear and thus movement of the vehicle is prevented. While engaging the parking brake from its disengaged position in the stationary state, sometimes, according to the teeth interval or the ratio of the tooth width to the gap width, the tip of a tooth of the detent can be positioned on the tip of a tooth of the gear. This tooth-on-tooth position prevents the entire mechanical activation chain from reaching its end position for the engaged position. However, an extremely small movement of the vehicle is sufficient to snap the detent into the next tooth gap. In the tooth-on-tooth position, a path sensor would not recognize that the locking position has been reached.
It is also conceivable to position a path sensor directly on the hydraulic piston in order to monitor the piston motion. However, this requires great expense in terms of construction because either pressure-tight or pressure-compensating implementations for contact switches or a complex rod assembly connecting to the hydraulic piston would be necessary.
U.S. Pat. No. 5,370,449 describes an electrically activated friction-fit parking brake, in which a pressure sensor is used to determine whether the brake is depressed (low pressure) or released (high pressure). As soon as the pressure sensor reports a high pressure, which corresponds to a released brake, the preselected transmission gear is set. However, no sensor signal is used for the transition region of the braking effect, that is, between total braking effect and the released state, so that this region remains undefined and there is uncertainty in the transmission control. Thus, there is the risk that the transmission could enter a non-positive engagement before the brake is released. Also, the brake could be released before the transmission is engaged, so that on a slope, a vehicle not in gear could roll uncontrollably. With the control of positive-fit parking brakes, other problems occur beyond those of non-positive parking brakes.
Accordingly, an object of this invention is to provide a control device for the parking brake of a motor vehicle, by means of which the previously mentioned problems can be overcome.
A further object of the invention is to provide such a control device which is reliable and which enable vehicles to be equipped at a later time and in a simple way with an electrical control unit for a parking brake.
A further object of the invention is to provide such a control device which enables a freely selectable installation, has a simple construction, and is maintenance-friendly.
A further object of the invention is to provide such a control device which handles the system-dependent tooth-on-tooth position as a normal operating condition (engaged position) and enable diverse plausibility checks.
These and other objects are achieved by the present invention, wherein a control device for a parking brake includes at least one hydraulic cylinder and one pressure sensor that detects the pressure in the hydraulic cylinder directly or indirectly. The hydraulic piston of the hydraulic cylinder is connected mechanically to engagement elements of the parking brake, so that the parking brake is forced into an engaged or disengaged position in dependence on the hydraulic pressure. The pressure sensor outputs pressure signals that represent the state of the parking brake and that are evaluated by an electrical control unit. The pressure signals of the pressure sensor are evaluated relative to at least two predetermined pressure threshold values. A first pressure threshold value is defined to correspond to the engaged state of the parking brake. A second pressure threshold value, which is different from the first pressure threshold value, corresponds to the disengaged state of the parking brake. As a rule, the second pressure threshold value is greater than the first pressure threshold value.
The control or evaluation unit recognizes at least three pressure ranges: a lower pressure range, in which the parking brake is engaged, an upper pressure range, in which the parking brake is disengaged, and a middle transition range. An unambiguous return message about the instantaneous state of the parking brake activation is provided to the control device at all times. Signals are provided that enable a locking circuit to be designed so that the demands for high reliability are satisfied.
In connection with additionally available input signals, such as commands from the operator, output speed, solenoid valve voltage, ignition, engine speed, and the like, many different plausibility queries and error messages can be generated due to detection of the pressure range. The signals of the control and evaluation unit can be used for a return notification to the operator and/or for controlling the transmission. These can also be used to diagnose faults relative to hydraulic and/or electrical problems.
The pressure sensor can be mounted in a simple way directly to the piston chamber or to a channel that is connected to a connecting line running between the valve arrangement and the piston chamber of the hydraulic cylinder. Thus, the position to attach the pressure sensor can be freely selected. This permits a maintenance-friendly design that is simple in terms of construction. The electrical parking brake unit can also be retrofitted in a simple way. Preferably, a pressure sensor is chosen that is designed for the pressure pulses and pressure spikes of mobile hydraulics, and thus is adapted to rough operating conditions.
With application of the solution according to the invention, the tooth-on-tooth position mentioned above does not cause any problems because the control device recognizes, without any additional equipment, that the lower pressure threshold has been passed and that the parking brake has reached its engaged position.
The pressure sensor and the evaluation of at least two pressure thresholds enables an easy, reliable transition from the parked position to the driving position and vice versa in connection with continuously variable speed transmissions with a non-positive stationary state, without requiring the operator to perform a special engagement.
Setting of the pressure threshold values enables reliable vehicle operation. In particular, the transition range between an engaged and disengaged parking brake can be reliably recognized, so that faulty control of the vehicle transmission can be prevented. For example, during the transition from drive to park, the xe2x80x9cnon-positive stationaryxe2x80x9d state is maintained until the lower pressure threshold value has been passed.
In particular, in order to prevent problems that are connected with the tooth-on-tooth position described above, a preferred refinement of the invention is provided where the connection between the hydraulic piston and an activating mechanism for the parking brake is designed such that solely pressure forces are transmitted. For example, the activating mechanism contains an activating shaft that is connected to the hydraulic piston by means of a sliding connection. Normally, the activating shaft is pressed against the hydraulic piston by a main spring of the activating mechanism. In the case of an unpressurized cylinder and a tooth-on-tooth position, the hydraulic piston can assume its rest position while the activating mechanism is still extended, so that an intermediate space appears between the activating shaft and the hydraulic piston.
It is also an advantage to provide a compression spring that forces the hydraulic piston into its unpressurized position independent of the activating mechanism. If the activating rod assembly is blocked due to a tooth-on-tooth position, then the piston is returned due to lower pressure until it stops in the piston chamber under the action of the compression spring. First, the pressure in the piston chamber falls from a value, which can be derived from the spring force and the piston cross section, to zero or to the pressure level of the storage container. When it reaches the stop, the hydraulic piston allows setting of the parking brake. If a tooth gap becomes free due to a small movement of the vehicle, then the detent snaps directly and undamped into the gap.
Preferably, a main spring engages the activating mechanism of the parking brake, and this main spring forces the activating mechanism into its locked position and the hydraulic piston into its unpressurized position by means of an activating element.
Preferred refinements of the invention are focused on setting the first and second pressure thresholds. The first pressure threshold value depends on the design of the hydraulic piston and the action of at least one spring that forces the hydraulic piston into its unpressurized stop position against the hydraulic pressure generated by the valve arrangement. In the stop position, the spring exhibits a resting force that presses the hydraulic piston against a stop. The resting force of the spring is determined by its spring tension and corresponds to the hydraulic pressure in the piston chamber. The first pressure threshold is set so that it corresponds to a value between zero and the mentioned spring tension. That is, the first pressure threshold lies between the pressure zero and the pressure created in the piston chamber at the time at which the hydraulic piston moving into its unpressurized position has just reached its stop position.
For an engaged parking brake, the piston chamber is unpressurized. Bearing in mind economical batch production of a pressure sensor for which limited resolution in the range of smaller pressures is permitted, the lower threshold should be advantageously selected to be as large as possible. On the other hand, in the case of a tooth-on-tooth position, the piston chamber pressure should first pass below the lower pressure threshold value when the hydraulic piston has reached its stop in the piston chamber. Thus, it is advantageous to set the first pressure threshold so that it is clearly greater than zero and lies only slightly below the pressure value corresponding to the spring tension.
In order to guarantee sufficient allowance for reliability on either side, an advantageous design of the invention provides that the first pressure threshold be set to a value of 40% to 80% of the pressure value corresponding to the spring tension.
The second pressure threshold value preferably depends on the design of the hydraulic piston and the action of at least one spring, and lies between the spring pressure that corresponds to the spring force in the maximum disengaged position of the hydraulic piston and the hydraulic system pressure provided by the valve arrangement. Preferably, the second pressure threshold corresponds to a value of 60% to 90% of the power supply pressure or system pressure.
Preferably, the pressure sensor is an analog sensor and each voltage value of the output signal corresponds to the pressure in the piston chamber.
If the pressure value measured by the sensor is outside of the typical range, in other words, it is less than zero or greater than the value corresponding to the system or supply pressure, then the evaluation electronics determines that there is a defective pressure sensor. Advantageously, the pressure sensor can also include the possibility of diagnosing purely electrical errors, by means of which, e.g., a power interruption can be recognized. Here, an advantageous refinement of the invention is proposed because the voltage output signal of the pressure sensor is already greater than zero in the unpressurized state.